1. Field of the Invention
The present invention relates to clathrate compounds, high efficiency thermoelectric materials and thermoelectric modules utilizing the clathrate compound, the semiconductor materials or hard materials utilizing the clathrate compounds, as well as manufacturing methods thereof.
2. Description of the Related Art
Recently, in high-tech fields such as electronics, the development of new high performance materials which differ greatly from conventional materials has received much attention.
For example, various methods for using thermoelectric materials are under investigation, but conventional thermoelectric materials display poor thermoelectric conversion efficiency, and are limited to certain uses where reliability is not particularly important. Consequently, it has been deemed problematic to propose the use of thermoelectric materials for typical uses such as waste heat power generation.
Furthermore, in order to improve the efficiency of these type of thermoelectric materials and enable their use as high efficiency thermoelectric materials, the following types of conditions need to be satisfied.
(1) a low thermal conductivity
(2) a high Seebeck coefficient
(3) a high electrical conductivity
However, the technique employed for developing conventional thermoelectric materials has involved selecting a composition based on experience, and then pursuing development of that material. As a result, the only example of a thermoelectric material currently being developed, for which the value of the dimensionless figure of merit (ZT) is greater than 1 at temperatures above 700 K, is the p-type thermoelectric material GeTexe2x80x94AgSbTe2.
Furthermore in the semiconductor field, laser devices, which are essential to optical communication technology, use silicon (Si), germanium (Ge), or group III-V compound semiconductors such as gallium arsenide (GaAs). Because the temperature range for stable operation of this type of compound semiconductors is low, and ensuring good heat dissipation is a large problem, the development of the semiconductors which will also operate at higher temperatures has been greatly needed.
Furthermore, in the case of short wavelength laser emission devices required for use in high density recordings such as optical disks and digital video disks (DVD), semiconductors with a wide forbidden bandwidth are used. Examples of this type of wide forbidden bandwidth (wide gap) semiconductors include ZnS, ZnSe, GaN, SiC and diamond.
The emission wavelength of a semiconductor laser device is determined by the inherent forbidden bandwidth of the semiconductor materials, and if the emission wavelength and the forbidden bandwidth are termed xcex (nm) and Eg (eV) respectively, then the relationship is described by the equation (1) below.
xcex(nm)=1240/Eg(eV)xe2x80x83xe2x80x83(1)
The visible light region is between wavelengths of 380xcx9c760 nm, and the corresponding forbidden bandwidth is 1.63xcx9c3.26 eV. Conventionally, emission devices emitting green light and light further towards the red end of the spectrum, with wavelengths of at least 550 nm, have used group II-V compound semiconductors with a forbidden bandwidth of no more than 2 eV, such as GaP, GaAs, or GaAlAs.
However, in order to generate a blue light emission device with a wavelength of less than 500 nm, then from the relationship shown in equation (1) it is clear that a wide forbidden bandwidth (wide gap) semiconductor with a forbidden bandwidth of at least 2.5 eV is required. Examples of this type of wide gap semiconductor include group II-VI compound semiconductors such as ZnS (forbidden bandwidth: 3.39 eV) and ZnSe (forbidden bandwidth: 3.39 eV), group III-V compound semiconductors such as GaN (forbidden bandwidth: 3.39 eV), and SiC (forbidden bandwidth: 3.39 eV).
Furthermore in the field of hard materials, although diamond is widely used, because of the associated high cost, an alternative hard material has been sought after. Although cubic boron nitride (CBN) is able to be synthesized, it remains limited to applications such as abrasive grits, and a material which can be used for members in mechanical components and sliding components which require low friction and good abrasion resistance has been keenly sought.
If conventional materials are considered within the background described above, then first it is true to say that a thermoelectric material which satisfies all the requirements for a high efficiency thermoelectric material has not yet appeared. For example in the case of metals, although offering the benefits of a large electrical conductivity, they suffer from having a large thermal conductivity and a small Seebeck coefficient. In the case of semiconductors, although offering the advantages of a small thermal conductivity and a large Seebeck coefficient, because the electrical conductivity is small, they can not be considered a high efficiency thermoelectric materials. Furthermore it is known that BiTe is used as a thermoelectric material at around room temperature. However, the efficiency thereof at 100xc2x0 C. or higher is low, and it is unable to withstand practical use.
Furthermore, in order to use a thermoelectric material in typical power generation, a power generation system must be constructed by combining a p-type thermoelectric material and an n-type thermoelectric material. However, in the case of the aforementioned conventional thermoelectric material of GeTexe2x80x94AgSbTe2, an n-type material does not exist.
In contrast, with conventional semiconductor materials, the temperature range for stable operation is low, meaning the operating environment is limited to a thermal environment close to room temperature.
Conventionally, heat generation has been suppressed in order to achieve stable operation of a semiconductor device, and so a large heat radiator has been necessary. For example, widely used silicon devices typically have a stable operating temperature range below 125xc2x0 C., and so electronic equipment utilizing silicon devices has required large heat sinks. Even with the use of heat sinks, the stable operating temperature range for a silicon device is, at the most, no more than 200xc2x0 C., and currently semiconductor devices do not exist which are capable of withstanding use in fields such as automobile components, high temperature gas sensors, engine control sections of space rockets, underground detection measuring apparatus, and nuclear power applications. The circumstances are the same for compound semiconductors.
Furthermore, in order to use a compound as a semiconductor, a doping atom must be introduced to make the conductivity either p-type or n-type. However, in order to introduce a p-type or n-type doping atom into GaN or SiC, a new artificial superlattice structuring is necessary, which makes the crystal growth process difficult. Furthermore, ZnS suffers from an additional problem in that the crystals cannot be obtained cheaply. In addition, in the case of diamond, control of the doping atoms is problematic.
As a result, the reality is that a semiconductor with a wide forbidden bandwidth which can be operated stably under conditions of high temperature or high pressure is not currently available.
In addition, conventional methods of manufacturing clathrate compounds include a method disclosed in Japanese Unexamined Patent Application, First Publication No. Hei-9-183607, wherein a monoclinic system crystal is produced by heating a mixture of an element from group 4B of the periodic table and an alkali metal under an atmosphere of argon, while a cubic system crystal is produced by heating a mixture of an element from group 4B of the periodic table and an alkali earth metal under an atmosphere of argon, and the following mixing of the monoclinic system crystal and the cubic system crystal and subsequent heating to form a precursor comprising a ternary solid solution, this precursor was heated under reduced pressure to effect an alkali metal element distillation and produce the clathrate compound. According to this method, the production process is complex, and moreover because the clathrate compound is synthesized under conditions of reduced pressure, the rate of formation is slow and the yield is also poor.
Furthermore another method of manufacturing clathrate compounds is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei-9-202609, wherein a diamond anvil type high pressure apparatus is used, and graphite is subjected to 14,000 Mpa of pressure at room temperature for a period of one week to synthesize a clathrate compound. According to this method, because the clathrate compound is synthesized at room temperature, the rate of formation is slow and the yield is also poor.
However, the conventional methods described above either require a considerable length of time, or are complex resulting in a difficult synthesis, and moreover the yields are poor and only thin films have been successfully synthesized, and in fact the current reality is that no practical and viable method of producing clathrate compounds exists.
The present invention takes the above circumstances into consideration, with an object of providing a clathrate compound which displays promise as a high efficiency thermoelectric material, meeting the three conditions of a low thermal conductivity, a high Seebeck coefficient and a high electrical conductivity, and with a figure of merit which exceeds 1.
Another object of the present invention is to provide a superior thermoelectric material and thermoelectric module which may be used in typical applications such as waste heat power generation.
Furthermore, yet another object of the present invention is to provide a clathrate compound which may be used as a superior wide gap semiconductor which can be operated stably even under conditions of high temperature or high pressure. This type of wide gap semiconductor is ideal for applications such as blue light emission devices.
Furthermore, yet another object of the present invention is to provide a clathrate compound which may be used as a hard material in mechanical components and sliding components and the like, which require low friction and good abrasion resistance.
Yet another object of the present invention is to provide a simple and high yielding method of manufacturing, in bulk, a clathrate compound which displays the superior characteristics sought after in the various fields of application described above.
A clathrate compound of the present invention comprises a clathrate lattice with atoms of at least one element from group 4B of the periodic table as the main structure, doping atoms which are encapsulated within the lattice spacing of the clathrate lattice, and substitution atoms which are substituted for at least one portion of the atoms which make up the clathrate lattice. The doping atoms of the clathrate compound are atoms of at least one of the elements from group 1A, group 2A, group 3A, group 1B, group 2B, group 3B, group 4A, group 5A, group 6A, and group 8 of the periodic table, and the substitution atoms are atoms of at least one of the elements from group 1A, group 2A, group 3A, group 1B, group 2B, group 3B, group 5A, group 6A, group 7A, group 5B, group 6B, group 7B, and group 8 of the periodic table.
A clathrate compound of this type displays various superior physical, mechanical and electrical characteristics, and may consequently be used as a thermoelectric material, a semiconductor material, and a hard material.
In a clathrate compound of the present invention, it is preferable that the aforementioned doping atoms have a greater mass than the atoms which make up the aforementioned clathrate lattice. This suppresses vibration of the clathrate lattice atoms, and diffuses lattice vibrations thereby reducing the thermal conductivity, meaning the clathrate compound will display favourable characteristics as a thermoelectric material.
Furthermore, in a clathrate compound of the present invention, it is also preferable that the doping atoms have a smaller electronegativity than the atoms which make up the aforementioned clathrate lattice. This means electrons from the outermost shell of a doping atom can move readily to the atoms which make up the clathrate lattice, so that the overall compound displays metal-like characteristics, and displays favourable characteristics as a semiconductor material.
The present invention is a thermoelectric material made from the aforementioned clathrate compound. The thermoelectric material of the present invention has a low thermal conductivity, a high electrical conductivity, and a high Seebeck coefficient.
Furthermore, the present invention is also a thermoelectric module which utilizes a thermoelectric material made from the aforementioned clathrate compound. A thermoelectric module of the present invention has a low thermal conductivity and a high electrical conductivity, and also displays a superior Seebeck coefficient and figure of merit.
In addition the present invention is also a semiconductor material made from the aforementioned clathrate compound. A semiconductor material of the present invention has a wide forbidden bandwidth, and displays stable operation even at high temperatures.
In addition, the present invention is also a hard material made from the aforementioned clathrate compound. A hard material of the present invention has a hardness second only to that of diamond, and is able to be produced as a bulk material.
According to the present invention, a clathrate lattice described above may utilize a clathrate compound, silicon clathrate 46 (Si46), which is a mixed lattice of a Si20 cluster comprising a dodecahedron of Si atoms, and a Si24 cluster comprising a tetradecahedron of Si atoms.
According to the present invention, the aforementioned clathrate lattice may also utilize a clathrate compound, silicon clathrate 34 (Si34), which is a mixed lattice of a Si20 cluster comprising a dodecahedron of Si atoms, and a Si28 cluster comprising a hexadecahedron of Si atoms.
In addition, according to the present invention, the aforementioned clathrate lattice may also utilize a clathrate compound, carbon clathrate 46 (C46), which is a mixed lattice of a C20 cluster comprising a dodecahedron of C atoms, and a C24 cluster comprising a tetradecahedron of C atoms.
Furthermore, according to the present invention, the aforementioned clathrate lattice may also utilize a clathrate compound, carbon clathrate 34 (C34), which is a mixed lattice of a C20 cluster comprising a dodecahedron of C atoms, and a C28 cluster comprising a hexadecahedron of C atoms.
A first method of manufacturing a clathrate compound according to the present invention si a method wherein an elementary substance of the atoms required for constructing the aforementioned clathrate lattice, an elementary substance of the aforementioned doping atoms, and an elementary substance of the aforementioned substitution atoms are mixed together in a predetermined ratio, and following pressure formation into a desired form, are subjected to preliminary heat treatment, and then sintered using pressure sintering techniques to from the clathrate compound.
A second method of manufacturing a clathrate compound according to the present invention is a method wherein a compound of a group 4B element from the periodic table which incorporates at least one element to become the doping atoms and the substitution atoms is melted in an inert atmosphere, and following solidification, is cooled gradually, maintained at a temperature of at least 500xc2x0 C. for a period of at least 10 hours, and is then cooled further, before being washed to remove any excess doping atoms and substitution atoms, and subsequently pressure sintered (hot press or the like) to form the clathrate compound.
Because the raw materials are melted at high temperatures, the above method offers the advantages that the reaction proceeds reliably, the yield is good, and the product can be obtained in a relatively short time.
A third method of manufacturing a clathrate compound according to the present invention is a method wherein a compound of a group 4B element from the periodic table which incorporates at least one element to become the doping atoms and the substitution atoms is crushed, in an inert atmosphere, to a powder with a granular diameter of no more than 100 xcexcm, and is then spread into a thin film inside a heat resistant vessel and left to stand, and the following heating at a temperature of at least 500xc2x0 C. for a period of at least 10 hours, is then cooled, washed to remove any excess doping atoms and substitution atoms, and subsequently pressure sintered to form the clathrate compound.
Although requiring a long time for the synthesis reaction, large scale equipment is unnecessary and the method of manufacture is simple.
A fourth method of manufacturing a clathrate compound according to the present invention is a method wherein a fine powder of an intercalant graphite intercalation compound with a granular diameter of no more than 100 xcexcm and incorporating at least one element to become the doping atoms and the substitution atoms is spread into a thin film inside a heat resistant vessel and then left to stand, and the following heating at a temperature of at least 500xc2x0 C. for a period of at least 10 hours, is then cooled, washed to remove any excess doping atoms and substitution atoms, and subsequently pressure sintered to form the clathrate compound.
This method offers the advantage that because the element or elements of the doping atoms and substitution atoms are already incorporated within the intercalant graphite intercalation compound, doping and substitution can be conducted extremely easily.
The effects of the present invention are as described below.
A clathrate compound according to the present invention has properties resembling those of a metal, displaying favourable thermal conductivity, electrical conductivity and semiconductor like properties, and also possessing a high level of hardness.
A clathrate compound according to the present invention displays excellent applicability as a thermoelectric material, a semiconductor material, or a hard material.
Furthermore, with a method of manufacturing a clathrate compound according to the present invention, a superior clathrate compound which has properties resembling those of a metal, displays favourable thermal conductivity, electrical conductivity and semiconductor like properties, and also possesses a high level of hardness, can be obtained simply and with a good yield, through a comparatively simple manufacturing method.
That is, a clathrate compound of the present invention enables the provision of bulk hard materials, for which the incorporation of impurities is relatively simple in comparison with diamond.
Furthermore, a clathrate compound according to the present invention has a low thermal conductivity and a high electrical conductivity, and so is able to be used as a high efficiency thermoelectric material.
In addition, in the clathrate compounds according to the present invention, by controlling the impurity elements introduced and adjusting the energy band of the band gap, semiconductors with a wide band gap energy level can be obtained, which may be applied to blue light emission laser devices capable of achieving high recording densities. Furthermore, by offering stable operation across a wide temperature range, a clathrate compound of the present invention may also broaden the practical application of semiconductors.
More specifically, a clathrate compound of the present invention comprises a group 4B lattice as the basic clathrate lattice, and because doping atoms are encapsulated inside the clathrate lattice, vibration of the clathrate lattice is suppressed, the thermal conductivity is lowered and the electrical conductivity is increased. Furthermore, as a result of one portion of the atoms which make up the clathrate lattice being substituted with an element with 1xcx9c3 valence electrons, the lattice assumes semiconductor-like properties, thereby improving the Seebeck coefficient. Consequently, a material is obtained which is highly suitable as thermoelectric materials, having favourable thermal conductivity and electrical conductivity, and a favourable Seebeck coefficient.
In addition, thermoelectric modules which utilize thermoelectric materials of the present invention have a low thermal conductivity, a high electrical conductivity, and a high Seebeck coefficient, enabling the provision of thermoelectric modules with an excellent figure of merit.
In addition, a semiconductor which utilizes a clathrate compound of the present invention has a basic framework of a group 4B clathrate lattice, and this clathrate lattice is doped with atoms which have a smaller electronegativity than the atoms which make up the lattice, and so the original insulating properties of the clathrate compounds have been shifted closer to metallic properties to yield the properties of a semiconductor. Moreover, by substituting clathrate lattice atoms with atoms which have more, or fewer, valence electrons than the atoms which make up the lattice, p-type or n-type semiconductors can be produced. A clathrate compound semiconductor according to the present invention displays the wide forbidden bandwidth of the clathrate compounds, and so even at high temperatures the amount of leakage current is small, and stable operation can be ensured. Consequently, applications of the present invention to fields such as high temperature gas sensors, automobile control components, engine control components of space rocket engines, control components for nuclear power facilities, and underground detection measuring apparatus are also possible.
Furthermore, because it displays a wide forbidden bandwidth, a clathrate compound semiconductors of the present invention may also be used as a short wavelength blue light laser device.
Furthermore, the first method of manufacturing a clathrate compound semiconductor according to the present invention comprises the simple steps of mixing the simple constituent elements, and then following pressure formation, conducting preliminary heat treatment, and then pressure sintering to produce a clathrate compound with superior characteristics.
Furthermore, with the second method of manufacturing a clathrate compound according to the present invention, the rate of formation of the clathrate compound is faster than conventional manufacturing methods, and the yield is better, resulting in a simple, and efficient synthesis.
Furthermore, with the third method of manufacturing a clathrate compound according to the present invention, because special steps such as the melting step, which takes considerable time, are unnecessary, clathrate compound semiconductors can be obtained very simply.
In addition, the fourth method of manufacturing clathrate compound semiconductors according to the present invention uses an intercalant graphite intercalation compound which already incorporates the doping atoms, and so the formation of the carbon clathrate compound is simple, and the clathrate compound can be produced in a straightforward manner.